Thin-Film Sensors to Integrate Process Monitoring

In modern production processes, sensing is a critical issue, since more and more processes are becoming integrated into a closed control loop. In order to place the sensors closely enough to relevant locations in the production line they have to be as slim and compact as possible. At the Hannover fair, the Fraunhofer Adaptronics Alliance has demonstrated an innovative solution.

Information directly from the main stress zone of a manufacturing process is crucial for modern process control systems. Pressure and temperature data along with their associated spatial data make steady process control. To achieve exact data, the sensors must be attached to the work pieces -- but this is exactly where typically there is no room available for any sensors. For this reason the Fraunhofer Institute for Surface Engineering and Thin Films, of Braunschweig, Germany, has devised an innovative solution: The scientists are coating the machine tools with non-wearing thin-film sensors.

To withstand the abrasive forces in the main stress zones, the piezo and temperature sensors are enclosed in robust hydrocarbon layers. This makes it possible to integrate the sensors directly into tools for injection molding, sheet metal filling, and punching. The basis for these layered sensory systems is a multi-functional hydrocarbon layer, baptized DiaForce, which unites abrasive resistance with wear resistance and piezo-resistive behavior.

To achieve measurements with spatial information, the researchers deposited electrode structures made from chromium on top of the sensors. To prevent wear, these chromium layers are covered with an electrical insulator layer of SiCON, a material made of silicon and oxygen. Optionally, an additional meander structure can be applied to this SiCON layer, which would result in local temperature measurement devices. These devices again are protected by a SiCON cover. The entire system represents a layered sensor at an overall thickness of some 9 to 12 micrometres.

These layered sensors are attached to carriers shaped according to the respective application; afterwards they are integrated to the machine tool where they measure relevant parameter and transmit the results to a control unit. Thus they provide a significant contribution to understanding and monitoring the process. The measurement provides a significant contribution to process optimization and can checked against numeric simulation.

The sensor module for use in injection molding processes is a result of the "Wechselwirkungen" project funded by the German Ministry for Economy and Technology. The sensors will be on display at the Fraunhofer stand, Hall 2, Stand D13.